Solid state x-ray detectors known in the art work by collecting the charge liberated when an x-ray photon is absorbed within the active volume of the detector. An electric field is used to drive the charge towards a readout electrode and the induced signal on the electrode is essentially proportional to the energy of the incident photon. In a common configuration, there are two parallel electrodes at the front and rear of the device and a collimator on the front of the device that defines the entrance area for incident x-rays. A voltage difference applied between front and back electrodes provides the field to collect liberated charge. The front electrode is made thin enough so that x-rays easily penetrate through to the active volume where the field is established.
A photon is first absorbed by the photoelectric process within the active volume. The subsequent cascade of interactions produces a number of electron-hole pairs in proportion to the photon energy. Electrons and holes are swept in opposite directions by the field and induce a signal on the read-out electrode. During this process, the charge clouds expand by diffusion and electrostatic repulsion.
Solid state detectors have been made from high purity silicon (see for example U.S. Pat. No. 6,153,883) and other materials such as high purity germanium, CdTe, CdHgTe, CdZnTe can be used. Various materials and implanted atoms can be used to form the electrical contacts. Also, different shapes can be used to define the periphery of the detector. One well established type of detector is made from Lithium-compensated Si (so called “Si(Li)”) and detectors of this type with “top-hat”, “grooved” and “planar” geometry are known particularly from F. S. Goulding et al. (“Detector Background and Sensitivity of Semiconductor X-ray Fluorescence Spectrometers”, Advances in X-ray Analysis, Vol. 15, 1972, pp. 470-482).
For high sensitivity, it is desirable to have a large area for x-ray detection. However, when the active area of a solid state detector is increased, this increases the electrical capacitance between front and back electrodes. Rossington et al. (“Large Area, Low Capacitance Si(Li) Detectors for High Rate X-Ray Applications”, C. S. Rossington et al., IEEE transactions on Nuclear Science, vol. 40, No. 4, August 1993, pp 354-359), explain that electronic noise increases with increased detector capacitance so that detector energy resolution can be improved by reducing capacitance.
FIG. 1 shows a cross section of a typical grooved structure Si(Li) detector as described by Rossington et al or Goulding et al. FIG. 1 shows a section through the centre of the detector crystal 1 that is typically a circular disk a few mm in thickness. The front contact (electrode) 2 for these type of detectors typically involves a thin metal conductive coating and a number of options have recently been discussed by Cox et al (Nucl. Instr. and Meth. in Phys. Res. B, 241, (2005), 436-440). The rear contact (electrode) 3 is typically formed by diffusing Li into the silicon wafer prior to cutting out the shape of the crystal. The diffused Li region forms a conductive layer and a potential difference of typically a few hundred volts to a thousand volts is applied between the front and back contacts. This potential difference produces a semiconductor depletion zone 4 between the electrodes where the silicon behaves more like an insulator. The depletion zone extends to the front of the detector and throughout the Li-compensated silicon. The compensated region is typically limited in extent at the sides of the detector and the uncompensated silicon constitutes most of the undepleted silicon 5 around the periphery of the detector that is effectively conductive and connected electrically to the front contact. Within the depletion zone, there is a strong field, shown by arrows and equipotential field lines in FIG. 1. An x-ray (generally indicated at 6) passing through the aperture defined by the entrance collimator 7 penetrates through the front contact 2 to reach the depleted silicon and liberate charge. For a single x-ray photon absorbed near the front contact, the liberated charge cloud of electrons is swept towards the back contact 3. By the time it reaches the back contact, the charge cloud may have spread to reach dimensions of the order of 100 μm according to Goulding et al. The movement of charge in the field induces a signal on the back contact. The back contact is typically connected to the gate electrode of a field effect transistor in the first stage of a charge sensitive amplifier for the signal. The noise and resolution of such a detector is influenced by the capacitance between the back electrode 3 and the front electrode 2 which is effectively connected to the undepleted silicon at the periphery. For the example shown in FIG. 1 where the distance between front and back electrodes is about 3 mm and the back contact is about 6 mm in diameter, the capacitance is about 0.94 pF.
One approach to reducing capacitance is to reduce the size of the readout anode. Tikkanen et al (Nucl. Instr. and Methods, A 390, 3 (1997) 329-335) describe a Si(Li) detector where the anode side of the crystal has a smaller diameter to reduce the readout capacitance. This is shown schematically in FIG. 2 (with analogous reference numerals as for FIG. 1). For this device, x-rays entering at the extreme periphery of the device showed evidence of tailing due to poor charge collection but x-rays entering most of the area of the front face produce acceptable tailing. However, if the back contact anode is made much smaller, this continues to reduce capacitance but increases the region of poor charge collection. FIG. 3 shows the same detector design as for FIG. 1 but with a back contact 3 that is only 3 mm in diameter. In this case, the capacitance is reduced to 0.62 pF but there are now very weak field regions near the readout electrode indicated by “W” in FIG. 3. For a photon absorbed in these regions, there is no strong field to sweep the charge quickly towards the anode and therefore an increased chance that some electrons will be trapped or recombine before they are collected. If an x-ray is absorbed near the front contact 2, Goulding et al explain that the expanding electron charge cloud will be large by the time it reaches the back of the detector and some of the cloud may fall in this weak field region “W”. Either effect results in a signal measurement well below the correct value and this degraded measurement will appear in the tail or background on the low energy side of the spectral peak corresponding to the incident photon energy. Goulding et al show that poor charge collection can be improved compared to a “top-hat” detector by using an earthed “guard ring” around the anode. However, with this “guard ring” approach, the field is essentially the same as in FIG. 1 so charge liberated by x-rays incident on the front face opposite the guard ring at the back will not reach the anode. Thus, the active area on the front face is simply reduced in proportion to the reduction in capacitance.
In order to retain the large front active area for detection with a small detector capacitance, Rossington et al describe a detector with a small back electrode where the front face electrode is effectively extended around to the sides. FIG. 4 shows this schematically. Starting with a conventional grooved Si(Li) structure, a cylindrical piece is cut out of the centre. The Li-diffused back contact, is cut away to 0.5 mm depth to substantially reduce the diameter of the back contact. In addition, the front face is either beveled or radiused to remove sharp edges to form the shape shown in FIG. 4. A Pd p contact is deposited on the front and side surfaces to act as the front entrance window and the back surface outside the small anode contact is passivated by coating with polyimide to form the device shown in FIG. 4. By extending the contact around to the sides, this structure improves on that of FIG. 3 because there is now a strong field towards the readout anode even at the back of the device. However, there are many processing steps required and since the side walls of the final device are extremely fragile, this makes handling and assembly difficult in practice.
It is therefore desirable to provide a detector having improved performance and easier manufacturability with respect to that of the prior art.